Atlantis TDD (4) Working with Legacy Code

After all this prep-work, it’s time to actually work on the game. Which game? I’m going to use Atlantis 1, the original game that started it all. The movement code in it is really gross, and the code only compiles under DOS, so the first order of business is to clean it up a bit. The result compiles under Linux without warnings, and even if nothing else comes out of this project of mine, at least there is now an Atlantis 1 code base you can just use.

This is classic legacy code: It’s not very modular at all, and there are absolutely no tests. Before I make any changes, I want to make sure that I won’t break anything, so I am going to put the movement code under testing. For this, I’m extracting the movement code into a process_movement function. Next, I’m going to make sure all code paths in that function are tested at least once.

Since this test is only protecting the original Atlantis game against regressions, I can write it in terms of the Atlantis code base and its objects (and use r2->units directly, etc). As long as this test passes, I can be certain that regular unit movement works. This kind of protection gives enormous peace of mind to a programmer who is working with a code base that isn’t his own (or is son old he doesn’t remember writing it). A couple more tests around modifications in the Atlantis source needed to be written, you can find them in the atlantis_test.c source on github.

Next up, I’m going to slap together an interface to stitch the Atlantis 1 implementation into my interfaces! And maybe then I can write my movement logic?